The ability to transport a gas to or from the dissolved state in a liquid has many uses. These include purifying drinking water through ozonolysis, oxygenating bioreactors and restoring oxygen to blood; oxidizing volatile organic compounds in water; adding gaseous reactants to liquid chemical reactions and supplying oxygen and removing volatile pollutants from waste water to name a few.
Generally in conventional methods of gasifying a liquid, the gas is bubbled directly into the liquid. Devices such as perforated or fritted sparging tubes and nozzles may be used to reduce the size of the bubbles. Although bubble size reduction improves the rate of mass transfer by raising the gas-liquid interfacial area per unit volume, bubbling is highly inefficient for gasifying a liquid and has additional shortcomings. Due to contact inefficiency, bubbling normally requires adding more than the required stoichiometric amount of gas. Excess gas must be discarded or recovered. Furthermore, the discarded gas frequently may be an undesirable pollutant and before it can be emitted to the environment, the gas must be treated. At the very least, discarding excess gas adds material cost without adding value to the primary product. Recovery of excess gas also complicates the product manufacturing process which adds still more cost. Process complications introduced by the recovery of excess gas can include stripping entrained liquid or upstream contaminants from the exhaust gas and measuring the concentration of such liquid and contaminants in the recovered gas. Liquid entrained in the excess gas can contain dissolved solids which tend to precipitate in the gas recovery equipment. Removal of these solids further adds to the difficulty of recovering the excess gas.
Bubbling also can be incompatible with the process for which the liquid is being gasified. For example, in a bioreactor, the agitation caused by bubbling can interfere with growth of fragile cells or destroy the cells. Gas bubbles entrained in oxygenated blood can be dangerous to an individual and normally should be eliminated completely.
Gas permeable polymer membranes might present an attractive technology for conducting mass transfer of gases. U.S. Pat. No. 5,051,114 to S. Nemser, issued Sep. 24, 1991, which is incorporated herein by reference, teaches the use of permeable polymer membranes for enriching or separating a gaseous organic compound in a gas or a gas mixture. However, most gas permeable membranes are not suited to transporting gas to or from a liquid. If the membrane is perforated or porous, gas can pass through the membrane too quickly and bubble into the liquid with the attendant disadvantages noted above. Also, the liquid can leak through the perforations or pores to contaminate the gas. Additionally the liquid and/or solids which might be present can clog the pores to reduce gas transfer.
Most nonporous polymer permeable membranes present too great a barrier to gas transfer for practical gasifying or degasifying a liquid. Low free volume gas permeable membranes of nonporous polymers have wholly inadequate gas permeability. Other known high free volume, nonporous polymer, gas permeable membranes are not acceptable for transporting gas to or from a liquid. Polytrimethylsilylpropyne ("PTMSP") is one of few known high free volume, nonporous polymers potentially suitable for gas permeable membranes. When used to gasify liquids, PTMSP membranes yield initially substantial but rapidly and dramatically declining gas flux. Although there may be other explanations, it is understood that this flow rate reduction is caused by liquid filling the free volume and thereby obstructing gas flow. Furthermore, certain corrosive gases, such as chlorine and ozone, chemically attack PTMSP. Silicone rubber is another nonporous, polymer with potential use in gas permeable membranes. Unfortunately, silicone rubber cannot be fabricated easily into thin membranes or thin coatings on high surface area substrates. Consequently, silicone rubber membranes usually include a thick polymer layer which constrains gas flow to relatively low rates.
It is very desirable to provide a nonporous, permeable polymer membrane capable of transporting gas to and from the dissolved state in a liquid at high flow rates. According to the present invention it has been discovered that nonporous gas permeable membranes formed from certain copolymers of perfluoro-2,2-dimethyl-1,3-dioxole ("PDD") allow gas transfer into and out of a liquid at high rate. Furthermore, the high gas flux can be maintained for extended duration.
The present invention thus provides a method of transferring a gaseous component between two fluids having different partial pressures of the gaseous component, and wherein at least one of the two fluids is a liquid, the method comprising:
contacting one of the two fluids with a first side of a two-sided, membrane unit, the membrane unit including a membrane (i) being substantially impermeable to the liquid and having a permeability to oxygen of at least 100 barrers; (ii) formed from an amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole; and (iii) being at a temperature below the glass transition temperature of the amorphous copolymer; and PA1 simultaneously contacting the second side of the two-sided, membrane unit with the other of the two fluids. PA1 contacting blood with a first side of a two-sided, membrane unit, the membrane unit including a membrane (i) being substantially impermeable to blood and having a permeability to oxygen of at least 100 barrers; (ii) formed from an amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole; and (iii) being at a temperature below the glass transition temperature of the copolymer; and PA1 simultaneously contacting the second side of the two-sided, membrane unit with a gaseous mixture containing oxygen at a partial pressure higher than the low blood oxygen partial pressure. PA1 contacting the liquid reaction medium with a first side of a two-sided, membrane unit, the membrane unit including a membrane (i) being substantially impermeable to the liquid reaction medium and having a permeability to oxygen of at least 100 barrers; (ii) formed from an amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole; and (iii) being at a temperature below the glass transition temperature of the copolymer; and PA1 simultaneously contacting the second side of the two-sided, membrane unit with a gaseous mixture containing oxygen at a partial pressure higher than the low oxygen partial pressure.
In one aspect this invention further provides a method of oxygenating blood having a low blood oxygen partial pressure, the method comprising:
In another aspect pertaining specifically to a bioreactor, the present invention additionally provides a method of oxygenating a liquid reaction medium containing living cells and having a low oxygen partial pressure, the method comprising: